1. Field of the Invention
The present disclosure relates to an image processing apparatus and an image processing method for correcting a color of an image output from a printer.
2. Description of the Related Art
In response to the need for outputting documents and images in color, which has been increasing in recent years in keeping with the trend of computerization, various types of printers have emerged. Types of color image formation include dye sublimation, thermal transfer, and inkjet, but electrophotography is considered to excel in speed of the image formation.
Image forming apparatuses employing electrophotographic methods, however, suffer from significant variation in image density depending on operating temperature and humidity, characteristic variability of a photosensitive body and a developing agent, and the durability of a developing device and the like. Color image forming apparatuses, in particular, present additional disadvantage of change in color.
To solve these problems, traditional electrophotographic apparatuses employ calibration techniques to create a one-dimensional LUT (look up table) for density correction corresponding to a “single color” of cyan, magenta, yellow, and black (hereinafter referred to as C, M, Y, and K, respectively). An LUT is a table that represents output data corresponding to input data partitioned by a specific interval and allows description of a non-linear characteristic, which cannot be described by an arithmetic operational expression. The one-dimensional LUT for density correction is a table that includes an output signal value corresponding to each input signal value of C, M, Y, and K. A toner is used by an amount corresponding to the output signal value to form an image on paper.
In order to create a one-dimensional LUT, a chart including data of different densities corresponding to each toner of C, M, Y, and K is output by a printer.
This chart is then measured by a scanner, a colorimeter, or the like. Measured values are compared against predetermined target data to create a one-dimensional LUT for density correction for each of C, M, Y, and K independently. This processing is called single color calibration. A single color calibration is executed to correct a single color reproduction characteristic, such as a maximum density and a tone characteristic.
It is, however, difficult to guarantee a “multi-color” by adjusting the single color density characteristics with the one-dimensional LUT, because a multi-color, including a plurality of toners such as red, green, blue, C, M, and Y, involves a non-linear difference depending on a printer. As a solution to this, a calibration technique has been proposed in which a chart, created with a multi-color within a range that can be output by a printer, is output by the printer and then measured by a scanner or a colorimeter for a comparison against a predetermined target value to arrive at a correction value (see Japanese Patent Application Laid-Open No. 2011-254350). In this document, a technique has been disclosed in which a destination profile, among ICC profiles, is modified to correct a color difference of a multi-color. An ICC profile is data, defined by ICC (International Color Consortium), to be used for a color conversion. In this technique, a chart, created with a multi-color, is output by a printer and then is measured by a scanner or a colorimeter. A result of the measurement and a predetermined target value are used to arrive at a difference. The difference is used to update a three-dimensional LUT (destination profile) to correct the multi-color. The three-dimensional LUT is for converting a device independent color space (L*a*b*) of the ICC profiles into a device dependent color space (CMYK). This processing is called multi-color calibration. A multi-color calibration is executed to correct a color reproduction characteristic for a multi-color that is described by combining (overlaying) a plurality of color toners. L*a*b* is a device independent color space, with L* denoting brightness and a*b* denoting hue and saturation. It is desirable that the single color calibration be executed to correct a single color density before the multi-color calibration is executed. In some cases depending on a state of a printer, however, a multi-color may be more apt to vary than a single color, and, hence, executing the multi-color calibration alone may provide a sufficient result of correction. For example, a user with ample opportunity to output data of a “multi-color,” such as a photograph, is likely to obtain a sufficient result of correction by merely executing the multi-color calibration.
It is difficult for a general user, however, to make an appropriate judgment on whether both calibrations should be executed or the multi-color calibration should be executed alone when giving an instruction to execute calibrations. As a result, the user would execute both calibrations too often, which causes a series of actions including outputting and scanning a chart to be executed more than once. This results in excessive time and effort taken for the calibration.
As a solution to the challenge that the calibrations are executed too often as described above, a technique has been proposed in which a timing for executing a calibration is appropriately set (see Japanese Patent Application Laid-Open No. 2004-69803). This technique decides the timing of a subsequent execution of the calibration on the basis of the number of sheets printed after a previous execution of the single color calibration and before a present execution of the single color calibration, and on the basis of a density difference detected during the executions of the calibration. This can suppress density variations and simultaneously optimize the number of executions of the density control.
The related art, however, discloses the technique that is merely concerning the execution of one type of calibration, which is the single color calibration. A user, thus, cannot make an appropriate judgment on which calibration should be executed when more than one type of calibration technique, which are the single color calibration and the multi-color calibration, can be executed independently from each other.